“Understanding superalloys on the atomic scale”
As conventional Ni-based single crystal superalloys are reaching their temperature capability limits, novel design concepts or alloy systems are critically required to meet the next-generation jet engine temperature specifications. Better performing alloys facilitate more efficient engine operations, which ultimately results in energy and cost savings, and lower carbon footprint. However, predicting the mechanical performance of parts in service requires understanding of their deformation behavior and the associated dynamic microstructural evolution down to the near-atomic scale. In this talk, I will showcase the use of atom probe tomography to reveal the three-dimensional chemistry of different crystal defects and its interplay with the alloy composition and service conditions. Through three examples, I will show how solute atoms in Ni-based superalloys interact with different types of crystal defects generated by plastic deformation to completely govern the microstructural evolution and mechanical behavior. During creep, atomic segregation to shearing dislocations is strongly dependent on the applied stress and defect type. The ensuing mass-transport of solutes during dislocation motion results in local phase transformations leading to a global microstructural evolution. Furthermore, different local phase transformations can also occur along planar faults, depending on the alloy composition, and influence the mechanical properties. Alloys with very slight compositional variations can show marked differences in creep performance. The atomic scale characterization revealed that the superlattice extrinsic stacking faults easily transitioned to deleterious microtwins when the parent crystal structure is stable. The segregation to deformation twin boundaries is also especially detrimental as it decreases the ductility of the alloy and hence leads to pre-mature failure. Meanwhile, a metastable parent crystal structure can lead to the formation of stable four atomic layer thick intermetallic phases along the planar faults, preventing further shearing. From these observations, an atomic-scale driven alloy design approach is proposed that exploits phase metastability to controls and promote local phase transformation along planar faults, aimed at designing superalloys with enhanced creep resistance.